Current Flag

It is a yellow over red horizontal bicolour with centred arms
Source: this online catalogueKlaus-Michael Schneider, 18 Aug 2017

Coat of Arms

Description of coat of arms:
Shield parted per fess; above Argent, a demi-eagle issuant Sable, crowned and armed Or and tongued Gules; beneath quartered of Or and Gules.
Meaning:
In the 13th century the New Town (Neustadt) used seals displaying St. Lambert, while the Dammstadt used seals displaying St. Stephen and St. Nicholas. The Old Town (Altstadt) had a proper seal since 1217 displayed bishop St. Gotthard of Hildesheim between two embattled towers, all topped by the five towers of the cathedral. A simpler version was also displayed in a secret seal from the 14th century. At the same time the quartered shield was used in seals and architectural sculptures. The current shield with the Imperial Eagle was granted in 1528 by Emperor Karl V. The shield was topped by a helmet Argent with scarves of red and yellow and crested by a virgin issuant, dressed of the same, wearing a beret with plumes of red and yellow. She was holding a rosary in her hands. She was considered to be Fru Veye, a woman, who appeared every year on the Whitsuntide Festival. Others call her Hildesia, which would be an incarnation of the city disguised as a woman.
Source: Stadler 1970, p.49
Santiago Dotor, 8 Mar 2002

and Klaus-Michael Schneider, 18 Aug 2017

Plain Flag reported 1970

3:5 image by Stefan Schwoon, 23 Mar 2001

The city used a plain yellow-red bicolour. Hildesheim is one of the oldest cities in Lower Saxony and nowadays the seat of a county. Although destroyed in World War Two, the city has many cultural treasures. Two of its churches, the cathedral and St. Michael's church are part of Unesco's World Cultural Heritage list.
Stefan Schwoon, 23 Mar 2001

Plain Flag reported 1891

1:2 image by Klaus-Michael Schneider, 22 Nov 2011

Description of flag:
The ratio is given as 1:2. It was a yellow over red horizontal bicolour. The information is confirmed by Stadler 1970, p.49.
Source:H.Ahrens "Hannoversche Landschafts-und Städtewappen", 1891, plate XX
Klaus-Michael Schneider, 22 Nov 2011